1. Field of the Invention
The present invention relates to a control circuit for a vehicle to process signals useful in a vehicular collision avoidance system, more particularly, of the SECANT type for aircraft.
2. Description of the Prior Art
The present invention is concerned with improvements to and extensions of the signalling system disclosed and claimed in the above-identified copending patent applications relating to the basic principles of SECANT, which is a signalling system for avoiding midair collision between aircraft.
A detailed description of the basic principles of SECANT are disclosed in U.S. Pat. No. 3,755,811, issued Aug. 18, 1973, entitled "Discriminating Signalling System" and U.S. Pat. No. 3,803,608, issued Apr. 9, 1974, entitled "Separation Control of Aircraft by Non-Synchronous Techniques," both based on the inventions of Jack Breckman, assigned to the common assignee as the present application.
In air traffic control systems, it is advantageous if not essential to provide a means of determining the relative if not the absolute altitudes of aircraft in the vicinity of each respective aircraft. Various techniques are known for providing such altitude information as by the use of converting barametric altimeter or radar altimeter information into appropriate signals for transmission to cooperative aircraft and ground stations and for comparing comparable altitude information from remote aircraft and ground stations.
The problems concerning the determination of such altitude information becomes increasingly more acute as the speed of cooperative aircraft increases and the density of traffic caused by cooperative aircraft increases. The SECANT system described in detail in the above patents and copending applications is based on a statistical correlation of an exchange of interrogation and reply signals that provides a means for determining range and tau (the ratio of range to range rate which is an indication of possible or potential time to collision) between the cooperating aircraft. The SECANT system utilizes two basic discriminants to achieve a high order of accurate correlation with a minimum chance of statistical error. The discriminants are based on the concept of frequency hopping and frequency jitter. Frequency hopping is achieved by providing a plurality of different signal frequencies for transmitting and receiving signals between the aircraft. Frequency jitter provides for the discriminant that minimizes the chance of false signals being received in the correlation of the frequency-hopped signals by jitter of the time of transmission relative to a fixed repetition rate of each signal.
In one prior art system as described in U.S. Pat. No. 3,603,993, issued Sept. 7, 1971, entitled "Control Apparatus," and assigned to Honeywell, Inc., for developing altitude information signals, there is provided means that utilizes a pulse position encoder wherein a first pulse is transmitted at a given time followed by a second pulse which is positioned relative to the first pulse proportional to the exact altitude of the encoding aircraft based on an altitude sensor. Such a sensor may be a barometric type or a radar type of sensor for absolute altitude. In such a system the means for positioning the second pulse, or to position the pair of pulses by techniques known as pulse position modulation (PPM), analog techniques in some portion of the control circuitry is required in order to develop a signal that is proportional to the exact altitude. Such a procedure requires instruments that are extremely sensitive to altitude variations and, in addition, some form of analog to digital conversion is required if the signal processing circuits are in digital form.